This invention relates to a method and apparatus for locating the position of a mobile station (MS) in a mobile telecommunications system.
It is desirable in certain applications for an operator of a mobile telecommunications system such as a cellular telecommunications system to be able to identify the geographical position of an MS during communication with a base transceiver station (BTS) of the system. For example, the US Federal Communications Commission (FCC) has implemented a position location mandate requiring system operators to be able to locate MS position with less than 125 m error in 67% of cases in order to provide an E911 (emergency) position location service in IS-95 based systems in the North American cellular and PCS bands.
A BTS may in principle locate the position of a MS by measuring the round-trip delay of a signal sent from the BTS to the MS and back (i.e. the elapsed time between transmission of a signal from the BTS and reception of the MS""s response) to evaluate the distance from the BTS to the MS, and by estimating the angular bearing from the BTS to the MS. To implement the FCC mandate, we can determine that we are required to measure signal RTD to an accuracy of approximately 560 ns, corresponding to a longitudinal (radial) position error of 84 m rms (root mean square). Similarly, we can tolerate a lateral (circumferential) position error, due to errors in the bearing estimate, of 84 m rms. So in an urban (up to about 1 km cell radius) scenario we require less than 4.8xc2x0 rms bearing error and in a rural environment (up to about 10 kM cell radius) less than 0.48xc2x0 rms bearing error.
With any radio-based direction-finding system there are several mechanisms by which we can potentially obtain errors in attempting to estimate the angle of arrival and range of received signal sources, namely:
Error mechanisms in measuring signal round-trip delay (RTD)
Noise/Interference
Multipath (angle and delay spread)
Doppler
Calibration Errors
An object of the invention is to enable a radio-based direction finding system to reduce errors due to Doppler frequency shift and spread, and carrier frequency offset, in particular relating to potential errors due to MS motion in a cellular telecommunications system.
A further object of the invention is to enable calibration of a radio-based direction finding system in a radio telecommunications system such as a cellular telecommunications system, for example in order to overcome errors due to imperfections in the reception equipment (antennas, cables, filters, radio chains, etc).
The invention provides in its various aspects a method and apparatus for estimating a bearing from a base station or cellsite to a mobile station using a direction finding antenna coupled to a receiver circuit located at the base station or cell site, and to the calibration of the direction finding antenna and the receiver circuit, as defined in the appended independent claims. Preferred or advantageous features of the invention are set out in dependent subclaims.
The invention in a first aspect may thus advantageously provide a method and apparatus for compensating for errors in an estimated bearing between a multi-element direction finding antenna and a mobile station due to motion of the mobile station or due to carrier frequency offset. These errors may arise as a bearing bias due to frequency shift, or offset, or due to Doppler frequency spread caused by the combination of mobile station motion and multipath, if signals received at the antenna elements are not sampled simultaneously. This aspect of the invention may advantageously permit compensation for these sources of error by estimating the Doppler frequency offset and/or spread and applying compensation to the phases of signals sampled from each antenna element before the signal complex phasors are used to estimate the bearing of the mobile station. In preferred embodiments, the Doppler frequency offset and/or spread are estimated by applying a time-domain Fourier Transform to signal samples successively received at the same antenna element.
The invention in a second aspect may thus advantageously provide a method and apparatus for calibrating the direction finding antenna and/or the receiver circuit. Several techniques for achieving this are proposed, each having different advantages, and it is further proposed to employ a method of selecting appropriate techniques in different situations or applications. All of these techniques find particular application to direction and/or range finding in cellular radio telecommunications systems.
In its second aspect, the invention may thus provide a technique for injecting calibration signals into the receiver circuit near the antenna (preferably as near to the antenna as possible), to calibrate the phase-sensitive direction-finding circuitry. It may also provide a technique for transmitting calibration signals from a near-field probe for reception by the direction finding antenna and processing by the receiver circuitry. This advantageously permits calibration of aspects of the antenna as well as the receiver circuit. The second aspect of the invention may further provide two types of beacon. A first type preferably emulates a mobile station but is fixed in position at a known location. A base station can therefore set up a call to the beacon, termed an autonomous beacon mobile, and allow the direction finding antenna and receiver circuitry at the base station to estimate the location of the beacon during the call. Comparison of the estimated and known locations, in terms of bearing and/or range, may then advantageously allow calibration of substantially all aspects of the performance of the antenna and the receiver circuitry. The second type of beacon is mounted at a cell site of a cellular telecommunications system and advantageously transmits a calibration signal for reception by any direction finding antenna situated at a neighbouring cellsite which has a footprint covering the beacon. The beacon may advantageously transmit a signal allowing calibration of bearing estimation functions of the direction finding antennas at neighbouring cell sites. In a CDMA system, for example, such a signal may be transmitted in a guard band of the transmission bandwidth. If the beacon signal is of narrow bandwidth, it may only be effective for bearing estimation. However, if a beacon signal of sufficient bandwidth can be transmitted in an available guard band, it may be used for range calibration and/or to calibrate receiver functions such as receiver filters. Calibration resolution based on a beacon signal of limited bandwidth may advantageously be improved by averaging over time.
Further advantageously, preferred embodiments of the invention may enable a direction and range estimation system which meets the FCC E911 mandate in as many environments as possible. To achieve the necessary bearing accuracy in rural cells, which may be of large radius, the invention advantageously provides in a further aspect a direction finding antenna comprising two widely-spaced, multi-element subantennas. The resolution of the individual subantennas is advantageously sufficient to remove bearing ambiguities arising from the wide spacing of the subantennas.